188 LYMPHATIC SYSTEM. 



ties in the ground-substance of the tissue containing flattened cells, 

 ■o-hich do not form a continuous vascular wall. And although the 

 spaces present a very close relation to the lymphatic vessels, they can 

 hardly be considered as actually opening into them by patent orifices, 

 for the lymphatics proper have a complete wall of flattened cells 

 united by a small amount of intercellular substance : at the same time 

 this thin film can offer but a very slight resistance to the passage of 

 fluid from the tissue into the vessel, or even to the passage of leuco- 

 cytes or migrating cells, which, as is well known, penetrate the at least 

 equally closed wall of the blood-vessels. 



It has been a question whether the cell-spaces of the connective tissue are in 

 every case and completely filled by the cells, or whether they (the spaces) may 

 in some cases be either devoid of cells altogether, or but partially occupied by 

 them : so that room is left for the free passage of fluid. On this point we would 

 remark that in many cases it is impossible to observe a diiference between the 

 forms of the cells as shown by the gold method, and those of the Kjmca as shown 

 by treatment with solution of nitrate of silver, so that in these, at least, no open 

 Ijnnph-passage can be said to exist ; but in other cases the spaces are relatively 

 larger, and here, no doubt, the part unoccupied by the contained cell may be filled 

 by fluid. In oedematous conditions of the tissue, the cell-spaces become somewhat 

 distended with serous fluid, and then in aU cases they appear distinctly larger than 

 the ceUs. So that we may conclude that the so-called saft-caniilchen- or lymphatic- 

 canalicular-system is in many cases rather potential than actual ; that is, where 

 the saft-caniilchen or lymphatic canaliculi (which correspond with the cell-spaces) 

 are completely filled by protoplasmic cells. Still, lymph can readily find its way 

 between the cells and the ground-svibstance by which they are closely sur- 

 rounded. In other cases the cells incompletely fiU the cavities, so as to leave a 

 freer passage for both fluid and migratory corpuscles. 



Valves.— The lymphatic and lacteal vessels are furnished with 

 valves serving the same ofiice as those of the veins, and for the most 

 part constructed after the same fashion. They generally consist of two 

 semilunar folds arranged in the same way as in the valves of veins 

 already described, but deviations from the usual structure here and 

 there occur. 



Valves a re not present in all lymphatics, but where they exist they 

 follow one another at much shorter intervals than those of the veins, 

 and give to the lymphatics, when much distended, a beaded or jointed 

 appearance. Valves are placed at the entrance of the lymphatic trunks 

 into the great veins of the neck. They are generally wanting in the 

 reticularly arranged vessels which compose the plexuses of origin 

 already spoken of ; so that fluid injected into one of these vessels runs 

 in all directions, so as to fill a greater or a less extent of the plexus, 

 and passes along the separate vessels which issue from it. 



The lymphatics of fish and naked amphibia are, generally speaking, 

 destitute of valves, and may therefore be injected from the trunks ; in 

 the turtle a few^ valves are seen on the larger lacteals which pass along 

 the mesentery, but none on those upon the coats of the intestine ; and 

 valves are much less numerous in the lymphatics and lacteals of birds 

 than in those of mammiferous animals. 



Orifices. — It was at one time a prevalent opinion among anatomists 

 that die lymphatic and lacteal vessels begin on various surfaces by 

 open mouths, through which extraneous matters are absorbed. This 

 was especially insisted on as regards the commencing lacteals in the 

 intestinal villi. That opinion "has been since given up ; but more 



